This invention relates to image switching apparatus and, more particularly, to such apparatus which controls the switching between input video signals by using a switch control signal that varies with respect to time, thereby producing special video effects as a function of the selection between the video signals.
Image switching apparatus is used in video broadcasting, displaying and recording to switch between one video signal representing one particular scene and another video signal representing another scene. For example, one scene may dissolve into another, or a new scene may replace a present scene by "shifting" it out of the video picture. Typically, image switching devices which are used to achieve the foregoing scene changes control the switching between video signals by using a switch control signal, also known as an image switching key signal, that changes over time.
The technique of "shifting" an old scene out of the video picture is known as "wipe switching." Typically, a new scene gradually replaces the existing scene by wiping the video picture with the new scene in a left-to-right or right-to-left direction. This is achieved by gradually replacing each horizontal line of the existing scene with a corresponding horizontal line of the new scene. As the duration of the horizontal line of the existing scene decreases while the duration of the horizontal line of the new scene increases, the "wipe" effect is achieved.
The image switching key signal used to produce this "wipe" effect may change between two conditions, or levels, one of which selects the existing video signal and the other selects the new video signal, with the location of this changeover gradually shifting over time. For example, if the image switching key signal changes near the end of each line interval and if the position at which time change occurs gradually moves toward the beginning of each line interval, the new video signal will begin to replace the existing video signal near the end of each line interval and gradually increase in a right-to-left direction. Thus, the visual effect will be displayed as a new image gradually replacing the existing image in the right-to-left direction. That is, the new image will "wipe" across the existing image in this right-to-left direction. Alternatively, if the image switching key signal occurs near the beginning of each line interval and then gradually shifts toward the end thereof, the video effect will appear as the new image wiping across the existing image in the left-to-right direction. Thus, the "wipe" effect in the resultant video picture is displayed as a new image which appears initially as a small area and gradually increases until it fully replaces the existing image.
Another example of "wiping away" an existing image with a new image is referred to as "noise wipe switching." In this technique, the image switching key signal is controlled by a noise signal so that the switching or image replacement appears somewhat random. Nevertheless, this random-like switching is controlled over time to gradually, yet fully, replace an existing image with a new image. Here, however, the new image initially appears as random noise within the existing image, but the small random-like areas of the new image gradually increase in size until they fully replace the existing image.
An example of a noise wipe operation will best be appreciated by referring to the waveforms of FIG. 6. FIG. 6A is illustrative of a noise signal waveform of the type that can be used to produce the image switching key signal. When the noise signal amplitude is less than a threshold level, the image switching apparatus selects one video signal; and when the noise signal amplitude exceeds the threshold level, the other video signal is selected. Since the noise signal varies in a random manner, the selection between the first and second video signals likewise appears to be random. But, as the threshold level against which the noise signal amplitude is compared decreases, a greater proportion of the noise signal will exceed that threshold and, thus, a greater proportion of the new video signal is selected. Accordingly, if the image switching key signal is assumed to be at a relatively high level ("1") when the noise signal exceeds the threshold level, and at a relatively low level ("0") when the noise signal is less than the threshold level, then the existing video signal is selected when the image switching key signal is a "0" and the new video signal is selected when the image switching key signal is a "1".
FIG. 6A illustrates the change, or decrease, in the threshold level over time. This threshold level changes within the range between L.sub.MAX, which is greater than the highest amplitude expected in the noise signal, and L.sub.MIN, which is less than the minimum noise signal amplitude. When the threshold level falls to a first reference level L.sub.REF1, the image switching key signal appears as shown in FIG. 6B. Here, it is assumed that a new video signal is represented as X.sub.b and the existing video signal is represented as X.sub.a. When the noise signal exceeds threshold level L.sub.REF1, the image switching key signal is a "1" and image X.sub.b is selected. However, when the noise signal falls below this threshold level L.sub.REF1, the image switching key signal is a "0" and the existing video signal is selected to produce the image X.sub.a.
FIG. 6C illustrates the image switching key signal produced when the threshold level is reduced to the reference level L.sub.REF2 ; and FIG. 6D illustrates the image switching key signal produced when the threshold level is reduced to the reference level L.sub.REF3. As the threshold level decreases over time, more of the new video signal is selected, and image X.sub.b gradually replaces image X.sub.a.
In implementing the noise wipe technique depicted in FIG. 6, various types of noise signal generators may be used. For example, a memory may be used to store and read out different types of noise signals, such as noise that may appear in a television tuner that is tuned to a non-broadcasting channel, thermal noise inherently generated by transistors, white noise, and the like. The noise signal derived from such stored noise representations is, more or less, random.
Unfortunately, the noise signal generated in a typical noise wipe switching arrangement exhibits poor auto-correlation. Consequently, there may be substantial changes in the noise signal produced from one video field interval to the next. These time changes produce undesirable flicker in the display as one image replaces another. It had been thought that, to eliminate such flicker, the noise signal stored in a memory must be read out repeatedly so that the noise signal produced in each field is substantially the same. This has the undesirable effect of eliminating the random quality of the image switching key signal.
Another drawback associated with previously proposed noise wipe switching apparatus is the general inability to change the noise pattern. Stated otherwise, if a video display of the noise signal is produced, random areas, some large some small, of undefined shape will be observed. It has not been thought possible heretofore to vary such shapes, and particularly the horizontal and vertical dimensions thereof.
These drawbacks of noise wipe switching arrangements have constrained the ability of an operator to artistically vary the manner in which one video image is replaced with another. Thus, creative expression in producing video displays has been somewhat limited.